FIG. 1 is exemplary of a known tuner and IF signal processing apparatus indicated generally by the reference numeral 10. This tuner and IF signal processing apparatus 10 is for use with analog signals of a given broadcast standard, such as NTSC, PAL, and SECAM. This application describes NTSC application of the invention as an exemplary embodiment.
The Tuner/IF system 10 comprises a tuner 12 (with RF input 14 and IF output 16), IF filter(s) 18, and IF signal processor 20. The tuner's IF output 16 is at the standard frequency (e.g., desired channel spectrum centered at 44 MHz, picture carrier at 45.75 MHz, and sound carrier at 41.25 MHz). The IF filter(s) 18, which usually consist of one SAW (surface acoustic wave) filter for inter-carrier IF systems or two SAWF for parallel picture and sound IF systems, pass the desired channel and reject all others. In the parallel system one SAW filter passes the desired sound signal only, and the other passes the desired picture signal only. In either case, the filter characteristics include a “Nyquist Slope” through the double-side-band region of the picture IF spectrum. The filtered signal is applied to a conventional IF processing circuit 20 which performs such general functions as demodulation, AGC generation and the like and provides a processed baseband video output signal to a video processing circuit. The video processing circuit performs conventional functions such as color demodulation and other functions such as brightness, hue and tint control and the like.
With the advent of digital television (DTV), and specifically digital terrestrial television such as HDTV (high definition television), television receivers and their corresponding tuner/IF systems which provide proper tuning and filtering required for processing both NTSC and DTV signals are needed.
FIG. 2 illustrates a modification of the NTSC only system of FIG. 1 to provide a tuner/IF system 22 able to be used for both NTSC and DTV reception. In FIG. 2, a tuner 24 is modified to provide reception of both NTSC and DTV signals. Conversion signals are appropriately selected such that both types of signals produce a common IF signal frequency (e.g., about 44 MHz). Two SAW filters 26 and 28 are coupled in parallel with the output of the NTSC/DTV tuner. SAW BPF #1 26 has specific requirements for DTV signal reception and processing while SAW BPF #2 28 has specific requirements for NTSC signal reception and processing. For example, both SAW BPF #1 26 and SAW BPF #2 28 have a center frequency of about 44 MHz. However, SAW BPF #1 has a flat passband response, while SAW BPF #2 has the characteristics described above for the IF Filter(s) 18 in FIG. 1. The DTV filtered signal is applied to Digital IF processing circuitry 30. The digital IF processing circuitry 30 provides the filtered and processed DTV signal to Digital Link (i.e., decoder) circuitry (not shown). The NTSC filtered signal is applied to NTSC IF processing circuitry 32. The NTSC IF processing circuitry 32 provides the filtered and processed NTSC signal to video processing circuitry (not shown).
Due to passband flatness requirements for DTV signals, the DTV/NTSC tuner 24 in FIG. 2 has a wider bandwidth than the NTSC tuner 12 in FIG. 1. Consequently, the NTSC adjacent channel rejection of system 22 in FIG. 2 is not as good as system 10 in FIG. 1.